Test instrument for testing asymmetric digital subscriber lines

ABSTRACT

A method of testing ADSL (asymmetric digital subscriber line) circuits is provided. A test instrument is connected to the customer premises end of the ADSL circuit, consisting of an ATU-C modem on the central office end and an ATU-R modem on the customer premises end, with a twisted-pair telephone line connecting the ATU-R and ATU-C modems. A remote test instrument is coupled to the ATU-C modem, typically on a semi-permanent basis in the central office, dedicated for testing multiple ADSL circuits by communicating with multiple ATU-C modems via a switch or router. The test instrument and remote test instrument communicate with each other in full duplex via the ADSL circuit using Internet Protocol (IP) data packets. The test instrument generates the upstream data traffic, controls the test sequence, and controls and coordinates the throughput test with the remote test instrument. The remote test instrument sends the downstream data traffic and returns the results of the throughput test in the form of frame counts from its end of the ADSL circuit back to the test instrument at end of the test sequence. The results from the upstream and downstream throughput tests are then visually displayed to the user of the test instrument.

BACKGROUND OF THE INVENTION

This invention relates generally to test instruments for testingcommunication networks and in particular to a test instrument and methodfor testing asymmetric digital subscriber lines (ADSL).

ADSL is a new modem technology which converts existing twisted-pairtelephone lines into access lines for high speed digital communicationand multimedia services such as video on demand (VOD). ADSL operatesaccording to a frequency division multiplex (FDM) scheme in which thefrequency spectrum between 0 and 4 kiloHertz (kHz) is allocated for POTS(plain old telephone service) and between 4 kHz and 1.1 MegaHertz (MHz)is allocated for data. In this way, the huge installed base of twistedpair copper wire telephone lines may be used to carry both voice anddata traffic. By separating the voice and data paths, the telephoneswitching system is freed up for conventional voice traffic while thedata traffic is moved to digital networks, at data rates 50 times higherthan the conventional analog modems over POTS circuits. The POTS circuitis uninterrupted even if the data portion of the ADSL circuit fails.

ADSL is defined according to the American National Standards Institute(ANSI) as ANSI Standard T1.413. An ADSL circuit consists of an ADSLmodem on each end of the twisted-pair telephone line. At the telephonecompany central office, the modem is commonly called the ATU-C (ADSLtransmission unit--central) while the modem on the customer premise sideis the ATU-R (ADSL transmission unit--remote). ADSL provides data ratesup to 8 Megabits per second (Mbps) from the central office to thecustomer (downstream) while providing upstream data rates from thecustomer premises up to 640 kbps. This asymmetric relationship betweendownstream and upstream data rates matches the original intent of ADSLto provide high bandwidth multimedia services downstream with morelimited bandwidth requirements upstream from the customer premises.

Twisted-pair telephone lines that will be used for ADSL were designedfor low frequency analog voice service, commonly known as POTS. For aninstalled base of telephone lines, there will be substantial variationsin the distance from the customer premises to the central office, thediameter of the copper wires, as well as the number of bridge taps andload coils along each telephone line. ADSL commonly employs DiscreteMultitone (DMT) line coding which allows the ATU-R and ATU-C modems todynamically adapt to the line conditions to obtain the maximumthroughput through the ADSL circuit. In addition, ADSL modems may employforward error correction, trellis encoding, echo cancellation, and othertechniques to obtain lower error rate communications, particularly forerror sensitive applications such as video transmission. Because of itsability to dynamically adapt to the line conditions of the twisted-pairline, an ADSL circuit must be fully evaluated using traffic generationboth upstream and downstream to stress the ADSL circuit to obtainthroughput measurements as an overall measure of its performance.

Several types of test instruments exist that may be used in ADSLtesting, including line qualification testers and personal computersrunning test software. Line qualification testers are analog instrumentsthat test the physical condition of the twisted pair line, typicallyproviding such time domain reflectometer (TDR) information as well asattenuation versus frequency and d.c. resistance. However, unless thereis a problem with the twisted-pair line that prevents the ADSL circuitfrom operating properly, the installer of the ADSL circuit cares onlyabout the maximum available throughput through the working ADSL circuit,in units of bits per second (bps), that can be obtained which may becompared against a level of service guaranteed to the customer.

In a conventional digital circuit, the upstream and downstream datarates are the same, allowing a digital loopback test to be performed inwhich traffic is echoed back from the far end of the digital circuit.However, in an ADSL circuit, the upstream and downstream data rates aredifferent, making the digital loopback test unusable. Furthermore,conventional personal computers having a network interface card (NIC)typically lack the ability to generate traffic at a sufficient number ofpackets per second to create the level of stress required for thisthroughput measurement, creating the need for specialized trafficgenerator circuits capable of generating large amounts of packets persecond.

The upstream and downstream data paths in the ADSL circuit, althoughoperating according to the frequency division multiplexing scheme, mayinteract and interfere with each other to reduce the maximum availablethroughput. The throughput of the ADSL circuit therefore must bemeasured with traffic generated in both the upstream and downstreampaths simultaneously in order to stress the ADSL circuit, thereforerequiring two test instruments that are working on each end of the ADSLcircuit in tandem.

Therefore, it would be desirable to provide a test instrument capable oftesting an ADSL circuit, operating in tandem with a remote testinstrument at the opposite end of the ADSL circuit, to provide ameasurement of the throughput of the ADSL circuit.

SUMMARY OF THE INVENTION

In accordance with the present invention, a test instrument for testingADSL circuits is provided. An ADSL circuit consists of an ATU-C modem atthe central office end and an ATU-R modem at the customer premises end,with a twisted-pair telephone line connecting the ATU-R and ATU-Cmodems. The test instrument is connected to the customer premises end ofthe ADSL circuit. A remote test instrument is coupled to the ATU-Cmodem, typically on a semi-permanent basis in the central office,dedicated for testing multiple ADSL circuits by communicating withmultiple ATU-C modems via a switch or router.

The test instrument and remote test instrument communicate with eachother via the ADSL circuit using Internet Protocol (IP) data packets. Inthe preferred embodiment, the test instrument and ATU-R modemcommunicate via an Ethernet interface and the remote test instrument andATU-C modem also communicate via an Ethernet interface. The testinstrument and remote test instrument may vary only in their selectedroles as test instrument and remote test instrument, with the choice ofrole depending on which end of the ADSL circuit the respective testinstruments are connected. The test instrument generates the upstreamdata traffic, controls the test sequence, and controls and coordinatesthe throughput test with the remote test instrument. The remote testinstrument sends the downstream data traffic and returns the results ofthe throughput test in the form of frame counts from its end of the ADSLcircuit back to the test instrument at end of the test sequence. Theresults from the upstream and downstream throughput tests are thenvisually displayed to the user of the test instrument.

In addition to throughput, the test instrument according to thepreferred embodiment may also have the ability to measure connectivity,meaning the ability to communicate with network devices on the centraloffice side of the ADSL circuit, such as a switch or router that iscoupled to the Internet or Wide Area Network (WAN). Connectivity may beestablished using the ICMP (Internet Control Message Protocol) PING,either singly or in a series of PINGs over time to establish thereliability of the connection from the test instrument to the networkdevice. For each PING, the targeted network device is requested toreply. The remote test instrument can be PINGed in this manner to testthe connectivity of the ADSL link.

The test instrument according to the preferred embodiment may also havethe ability to measure latency, meaning the time lag associated withdata as it travels through the LAN. Latency may be measured bysynchronizing the clocks between the test instrument and remote testinstrument, sending data packets, and measuring the time differencebetween the sent and arriving packets. A series of latency measurementsmay be conducted over a period of time in order to measure latencyjitter, which is the variability of latency over time.

One object of the present invention is to provide a test instrument fortesting an ADSL circuit.

Another object of the present invention is to provide a test instrumentwhich, in combination with a remote test instrument, provides fortesting the maximum throughput of an ADSL circuit.

A further object of the present invention is to provide a method fortesting the throughput of an ADSL circuit by simultaneously generatingdata traffic in both directions through the ADSL circuit at differentdata rates.

An additional object of the present invention is to provide a method fortesting an ADSL circuit coupling a test instrument and a remote testinstrument to opposite ends of the ADSL circuit which simultaneouslygenerate data traffic in both directions through the ADSL circuit.

Other features, attainments, and advantages will become apparent tothose skilled in the art upon a reading of the following descriptionwhen taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration (not to scale) of a typical installation of anADSL circuit;

FIG. 2 is a simplified drawing (not to scale) of a test instrument andremote test instrument applied in testing the ADSL circuit shown in FIG.1;

FIGS. 3A and 3B together comprise a flow diagram of the method oftesting the ADSL circuit; and

FIG. 4 is a simplified block diagram of the test instrument of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified drawing of a typical installation of an ADSLcircuit from a central office of a telephone company to a customerpremises using a twisted-pair telephone line 10. Because twisted-pairtelephone lines vary greatly by length, diameter of the copper wires,and the number of bridge taps and loading coils, among other factors,the throughput of ADSL circuits will vary widely. An ATU-R (ADSLtransmission unit--remote) 12 is connected to the customer premises endof the twisted-pair telephone line 10. The ATU-R 12 decodes the ADSLsignal to obtain data packets which are sent and received via a 10BASE-Tline to a client 14 at the customer premises. 10BASE-T is commonlyunderstood to be twisted-pair Ethernet. Network devices such as switchesand hubs (not shown) may be used to connect other clients and servers inthe customer premises. Other network and link level protocols, such as100BASE-T or Token Ring, may be readily substituted for the 10BASE-Tline.

The ADSL circuit allows the twisted pair telephone line 10 to be used tohandle a POTS circuit for the telephone 16 as well. The POTS circuitdoes not depend on the proper operation of the data paths of the ADSLcircuit.

In the central office, each ADSL circuit terminates in an ATU-C 18, withmultiple ATU-Cs 18 typically mounted in a rack 20. The ATU-C 18 decodesthe ADSL signal to obtain IP data packets which are sent and receivedvia a 10BASE-T connection and separates off the POTS circuit forconnection to the public switched telephone network (not shown). Each ofthe ATU-Cs 18 is connected to other network devices such as a switch 22which selectively switches the data packets on to other networks, suchas the Internet. During an installation of an ADSL circuit, thetwisted-pair telephone line, already installed and operating as a POTScircuit, is terminated with the ATU-R 12 and the ATU-C 18 at oppositeends.

In FIG. 2, there is shown a simplified schematic drawing of a testinstrument 100 operating in conjunction with a remote test instrument104 to test an ADSL circuit 102 according to the present invention. Thetest instrument 100 is connected to the ATU-R 12 via a patch cord 106.The ATU-R 12 and the test instrument 100 communicate via standard10BASE-T Ethernet ports. Similarly, the ATU-C 18 and the remote testinstrument 104 communicate via standard 10BASE-T Ethernet ports. It isdesirable that the installer or maintainer of the ADSL circuit 102 havethe ability to verify the throughput of the ADSL circuit 102 as well itsconnectivity to selected network devices in the central office, such asthe switch 22. Throughput is a measure of the overall performance of theADSL circuit 102, under predetermined levels of data traffic andaccording to the conditions of the twisted-pair telephone line 10.

The ADSL circuit 102 provides data paths in two directions--"upstream"from the customer premises to the central office, and "downstream" fromthe central office to the customer premises. The upstream and downstreampaths interact with each other to some extent, making it desirable totest throughput by sending data traffic in both directionssimultaneously to appropriately stress the ADSL circuit 102. Because theupstream and downstream paths are not at the same data rate, a digitalloopback test will not work. Rather, the test instrument 100 and theremote test instrument 104 must work in tandem to generate theassymetrical upstream and downstream traffic and evaluate the resultsaccording to an automated throughput test as explained in more detailbelow.

The primary concern of the ADSL circuit installer is in obtaining anacceptable downstream throughput while handling an expected level ofupstream traffic, typically consisting of TCP (transmission controlprotocol) acknowledgments sent in response to the downstream trafficreceived. The upstream traffic, for the purposes of the throughput test,is thus kept fixed at an expected data rate found according to areasonable amount of experimentation. The ratio of the downstream datarate to upstream data rates to downstream data has been found to be onthe order of 10 to 1 and as high as 20 to 1 in the majority ofapplications. For downstream data rates ranging from 1.5 to 8 Mbps, anupstream data rate in the range of 60 to 640 kbps may be chosen. It isdesirable to use the single upstream data rate in order to obtain asingle throughput measurement of the downstream data path for testpurposes. A series of upstream data rates may be tested with anattendant increase in test time to obtain more detailed throughputtesting.

FIGS. 3A-B collectively comprise a flow chart of the method of testingan ADSL network according to the present invention. In summary, thethroughput test, begins by accepting the variables MIN (minimumthroughput), MAX (maximum throughput), STEP (step size), and upstreamdata rate from the user. The throughput test first establishescommunications between the test instrument and remote test instrumentand coordinates the sending of data traffic through the ADSL circuit atselected data rates according to a remote traffic generator protocol.The remote traffic generator protocol is a set of commands and datatypes that is the basis for sending a traffic generation request.

The test instrument then begins testing throughput, with the data rateof the downstream path starting at the minimum data rate andincrementing the data rate up by the step size. The maximum throughputis the upper limit, typically determined according to the ability of thenetwork devices to handle the network traffic without significantdisruption to other network data traffic ("production networkfriendly"). Below this maximum throughput, the throughput test will stopwhen the percentage of packets getting through falls below apredetermined minimum threshold, such as 95%, to obtain a maximumavailable throughput. Upon completion of the throughput test, theresults, including the maximum available throughput of the downstreamdata path, are formatted and displayed to the user.

In FIG. 3A, step 200 labeled BEGIN THROUGHPUT TEST, the test instrument100 and remote test instrument 104 are connected to both sides of theADSL circuit 102 (shown in FIG. 2) for testing, such as during a servicecall or a new installation of the ADSL circuit 102. The test instrument100 executes an instrument control program to implement the presentmethod and to further control the operation of the remote testinstrument 104 via commands sent via the ADSL circuit 102. The testinstrument 100 and the remote test instrument 104 may be identicalinstruments or the remote test instrument 104 may be of reducedconfiguration with no user interface since the remote test instrument104 is remotely controlled.

In step 202 labeled DOWNSTREAM--ENTER MIN, MAX, AND STEP DATA RATES,UPSTREAM--ENTER DATA RATE, the user is prompted to enter the variablesthat determine the data rates that will be tested for the upstream anddownstream data paths. In the preferred embodiment, the downstreamthroughput is determined by testing at different data rates, startingfrom the lower limit MIN, such as 4 Mbps, incremented by an amount STEP,such as 1 Mbps, and going as high as the upper limit MAX, for example 7Mbps. The upstream data rate is kept constant at an expected data rate,such as 50 kbps. The method may be readily modified to test anycombination of upstream and downstream data rates in order to obtain ameasure of throughput according to specific requirements.

In step 204 labeled SEND ARP REQUEST TO REMOTE TEST INSTRUMENT, the testinstrument 100 seeks to establish communication with the remote testinstrument 104 via an ARP (address resolution protocol) request. The IPaddress of the remote test instrument 104 must be known so that the ARPrequest is addressed to it. In the typical scenario, the remote testinstrument 104 is located in the central office and its IP address isknown by the user.

In step 206 labeled ARP RESPONSE RECEIVED?, the test instrument 100waits to receive a reply from the remote test instrument 104 to its ARPrequest. If the ADSL circuit 102 is functioning well enough to providebasic connectivity, this connection should be made. If no reply isreceived, the test instrument 100 may continue sending the ARP requestand generate error messages according to step 208 labeled DISPLAY ERRORMESSAGE or terminate the throughput test because of a communicationfailure.

In step 210 labeled SEND TRAFFIC GENERATION REQUEST TO REMOTE TESTINSTRUMENT, once communications have been established, the testinstrument 100 now controls the operation of the remote test instrument104 and instructs the remote test instrument 104 to generate trafficthrough the downstream data channel at a specified data rate accordingto a remote traffic generator protocol, starting with MIN, and to countthe number of packets arriving from the upstream data channel duringtraffic generation. The test instrument 100 controls the remote testinstrument 104 according to the remote traffic generator protocol, a setof commands developed specifically for the application to allowefficient communication of MIN, MAX, STEP, along with synchronizing thestarting time of the traffic generation.

The UDP (user datagram protocol) is used to ensure that thecommunications between the test instrument 100 and remote testinstrument 104 are full duplex, with no automatic acknowledgmentstraveling back and forth, to ensure integrity of the throughput test.Because the communications between the test instrument 100 and remotetest instrument 104 are conducted according to the UDP, there is noautomatic acknowledgment of receipt as is available in TCP. Therefore,acknowledgments and replies are explicitly implemented in thecommunications to ensure that the instruments stay synchronized and arein communication during the traffic generation portion of the throughputtest.

In step 212 labeled ACKNOWLEDGED BY REMOTE TEST INSTRUMENT?, the testinstrument waits for an acknowledgment by the remote test instrument 104that the traffic generation request was received.

In FIG. 3B, step 214 labeled SIMULTANEOUSLY GENERATE UPSTREAM ANDDOWNSTREAM DATA TRAFFIC, the test instrument 100 and remote testinstrument 104 simultaneously generate upstream and downstream datatraffic at the selected data rates, with each test instrument countingthe number of packets received from the other, for a predeterminedperiod of time.

In step 216 labeled REQUEST FRAME COUNTS FROM REMOTE TEST INSTRUMENT,the test instrument 100, again according the remote traffic generatorprotocol commands, requests the frame count from the remote testinstrument 104.

In step 218 labeled RESULTS RECEIVED?, the test instrument 100 waits forthe reply from the remote test instrument 104 in response to the framecount request. If no response is received, the test instrument 100 mayrepeat step 216 or eventually terminate the throughput test because oflost communications.

In step 220 labeled FRAME COUNTS<THRESHOLD?, the test instrument 100receives the frame count from the remote test instrument 104 andcompares the frame count with the number of frames sent during thetraffic generation sequence of step 214 for each of the downstream andupstream data paths. The threshold is defined to be the minimumacceptable ratio of frames received to frames sent and may be differentfor the upstream and downstream paths, depending on the applicationneeds. In the present invention, the threshold for the downstream pathdetermines the throughput rate and is typically 90% to 95% by industryconvention. This threshold may be arrived at by reasonableexperimentation or determined according to the application needs. Thethreshold for the upstream data path is used only to ensure the upstreamlink is functioning properly, since its data rate is set at a nominalexpected value in order to stress the downstream data path.

If the percentage of frames received to frames sent falls below thethreshold, the traffic generation phase of the throughput test isterminated, since the maximum supportable data rate has been determinedto be the last downstream data sent before the present data rate.

In step 222 labeled DOWNSTREAM DATA RATE<MAX?, the traffic generationphase of the throughput test continues at increasingly higher data ratesthrough the downstream data path as long as the threshold is exceeded upto the maximum data rate MAX that will be tested. If MAX has not yetbeen reached, in step 224 labeled INCREMENT DOWNSTREAM DATA RATE BYSTEP, the downstream data rate is increased by an amount STEP defined bythe user in step 202, and the traffic generation phase of the throughputtest continues at step 210 with a higher downstream data rate.

As MAX is reached, the traffic generation phase of the throughput testis terminated, since the maximum supportable data has been determined tobe at least MAX, which was defined by the user in step 202.

In step 226 labeled FORMAT AND DISPLAY RESULTS TO USER, the results ofthe throughput test are displayed in terms of the maximum supportabledata rate downstream for a given upstream data rate.

In FIG. 4, there is shown a simplified block diagram of the testinstrument 100 shown in FIG. 2. The test instrument 100 is connected tothe ATU-R 12 via a 10BASE-T Ethernet connection in which IP data packetscan be sent full duplex using TCP or UDP protocols. Received frames fromthe ATU-R 12 are coupled to a frame processor 120 which operates toextract information from the received frames, such as the frame count.The frame processor 120 is implemented as a dedicated hardware processorin the preferred embodiment to gain processing speed but may also bereadily implemented in software according to performance requirements.

The frame processor 120 provides the frame information to amicroprocessor 122 which stores the frame information in a memory 124. Atraffic generator 126 coupled to the ATU-R 12 generates network trafficin the upstream direction through the ADSL circuit 102 at a duration anddata rate specified by the microprocessor 122. A user input 128 acceptsinput from the instrument user, via keypresses or by touching atouchscreen display. A display 132 displays the results of thethroughput test determined by the microprocessor 122 via a screen driver130. The display 132 comprises a touchscreen display in the preferredembodiment which graphically displays the information and allows foruser control via predetermined softkeys on the touchscreen display,which reduces the number of mechanical buttons and switches required onthe front panel of the test instrument 100.

The remote test instrument 104 has substantially the same block diagramas that of test instrument 100 in the preferred embodiment and isconnected to the central office end of the ADSL circuit 102. The rolesof remote test instrument and test instrument are readilyinterchangeable, depending on which end of the ADSL circuit 102 the userresides. Alternatively, the remote test instrument 104 could besimplified to delete the user interface components including the userinput 128, screen driver 130, and display 132, since the control of theremote test instrument 104 and user interface functions are performed bythe test instrument 100.

The remote test instrument 104 is controlled by the test instrument 100during the throughput test. Under remote control, the remote testinstrument 104 receives the data traffic generated by the testinstrument 100, obtains a frame count and simultaneously generates thedownstream data traffic at a selected data rate and time interval. Theframe count determined by the remote test instrument 104 is returned tothe test instrument 100 via the ADSL circuit 102.

It will be obvious to those having ordinary skill in the art that manychanges may be made in the details of the above described preferredembodiments of the invention without departing from the spirit of theinvention in its broader aspects. For example, other types of digitalsubscriber lines, commonly described as the xDSL family, may alsobenefit from the automated, full duplex, variable data rate method fortesting throughput according to the present invention. The ATU-R 12 andATU-C 18 may be incorporated into the test instrument 100 and remotetest instrument 104 in order to more directly characterize thetwisted-pair telephone line in terms of the ATU modem adaptationrequired and also provide more control over the types of error detectionand correction incorporated into the ADSL circuit 102. Other network andlink level protocols, including 100BASE-T for example, may be readilysubstituted for the 10BASE-T lines that couple the ATU-R 12 and ATU-C 18to other portions of the network. Therefore, the scope of the presentinvention should be determined by the following claims.

What we claim as our invention is:
 1. A method for testing an asymmetricdigital subscriber line, comprising:(a) coupling a test instrument to afirst end of said asymmetric digital subscriber line; (b) coupling aremote test instrument to a second end of said asymmetric digitalsubscriber line; (c) establishing communications between said testinstrument and said remote test instrument; (d) simultaneouslygenerating traffic in both directions through said asymmetric digitalsubscriber line, said test instrument generating traffic in a firstdirection for a first number of frames and said remote test instrumentgenerating traffic in a second direction for a second number offrames;wherein said remote test instrument counts the number of framesreceived from said test instrument to produce a first frame count, andsaid test instrument counts the number of frames received from saidremote test instrument to produce a second frame count; (e) calculatingthroughput results by comparing said first frame count with said firstnumber of frames and said second frame count with said second number offrames to determine the percentage of frames passing in both directionsthrough said asymmetric digital subscriber line; and (f) displaying saidthroughput results on a display of said test instrument.
 2. A method fortesting an asymmetric digital subscriber line according to claim 1further comprising displaying said throughput results for said firstdirection based on said first frame count and said first number offrames and for said second direction based on said second frame countand said second number of frames.
 3. A method for testing an asymmetricdigital subscriber line according to claim 1 wherein said first end islocated at a customer premises and said second end is located at acentral office.
 4. A method for testing an asymmetric digital subscriberline according to claim 1 further comprising:(a) incrementing a datarate of said traffic in said second direction until a ratio of saidsecond frame count to said second number of frames is less than athreshold value; and (b) obtaining a maximum available throughput ratebased on said data rate of said traffic in said second direction.
 5. Amethod for testing an asymmetric digital subscriber line according toclaim 1 wherein said test instrument and said remote test instrumentcommunicate in fall duplex according to a user datagram protocol.
 6. Amethod for testing an asymmetric digital subscriber line according toclaim 1 wherein said test instrument controls said remote testinstrument according to a remote traffic generator protocol.